1. Technical Field
This invention relates generally to measuring a parameter of a fluid and more particularly to a method and apparatus for measuring a parameter of a fluid such as velocity and volumetric flow rate of the flow within a pipe.
2. Description of Related Art
A fluid flow process (flow process) includes any process that involves the flow of fluid through pipes, ducts, or other conduits, as well as through fluid control devices such as pumps, valves, orifices, heat exchangers, and the like. Flow processes are found in many different industries such as the oil and gas industry, refining, food and beverage industry, chemical and petrochemical industry, pulp and paper industry, power generation, pharmaceutical industry, and water and wastewater treatment industry. The fluid within the flow process may be a single phase fluid (e.g., gas, liquid or liquid/liquid mixture) and/or a multi-phase mixture (e.g. paper and pulp slurries or other solid/liquid mixtures). The multi-phase mixture may be a two-phase liquid/gas mixture, a solid/gas mixture or a solid/liquid mixture, gas entrained liquid or a three-phase mixture.
Currently, various sensing technologies exist for measuring various physical parameters of fluids in an industrial flow process. Such physical parameters may include, for example, velocity, volumetric flow rate, composition, gas volume fraction, consistency, density, and mass flow rate. One such sensing technology is described in commonly-owned U.S. Pat. No. 6,609,069 to Gysling, entitled “Method and Apparatus for Determining the Flow Velocity Within a Pipe”, and U.S. Pat. No. 6,889,562, which are hereby incorporated herein by reference in their entirety. The '069 patent describes a method and corresponding apparatus for measuring the flow velocity of a fluid in an elongated body (pipe) by sensing vortical disturbances convecting with the fluid. The method includes the steps of providing an array of at least two sensors disposed at predetermined locations along the elongated body, wherein each sensor samples the pressure of the fluid at the position of the sensor at a predetermined sampling rate. The sampled data from each sensor at each of a number of instants of time spanning a predetermined sampling duration is accumulated and at least a portion of a so called k-ω plot is constructed from the accumulated sampled data, wherein the k-ω plot is indicative of a dispersion relation for the propagation of acoustic pressures emanating from the vortical disturbances. A convective ridge in the k-ω plot is identified and the orientation of the convective ridge in the k-ω plot is determined. The flow velocity based on a predetermined correlation of the flow velocity with the slope of the convective ridge of the k-ω plot may then be determined from this information. See also related technology disclosed in U.S. Pat. Nos. 7,673,524 and 7,895,903, which are hereby incorporated by reference.
Such sensing technology is effective in determining various parameters of a fluid flow within a pipe. However, as with any computationally complex process, there remains a desire to increase computational efficiency, accuracy and robustness.
For example, the known technique is based on an array of two or more strain sensors clamped to the outside of a pipe. Typically, these sensors are flexible piezo-electric polymer strain sensors fabricated out of PVDF. A plurality of sensors is clamped circumferentially around the pipe at a known spacing. The strain signature received by a first sensor is correlated to subsequent downstream sensors to determine the velocity of the flow. It was found that the signal to noise was dependent upon the number of sensors, and therefore processing channels, used. As the number increases the complexity and cost of the instrument also increases. In view of this, there is a need to improve the known technique, including improving the signal to noise ratio in the known signal processing.